sshd_config(1) — OpenSSH SSH daemon configuration file



  • SSHD_CONFIG(5)		    BSD File Formats Manual		SSHD_CONFIG(5)
    
    NAME
         sshd_config — OpenSSH SSH daemon configuration file
    
    SYNOPSIS
         /etc/ssh/sshd_config
    
    DESCRIPTION
         sshd(8) reads configuration data from /etc/ssh/sshd_config (or the file
         specified with -f on the command line).  The file contains keyword-argu‐
         ment pairs, one per line.	Lines starting with ‘#’ and empty lines are
         interpreted as comments.  Arguments may optionally be enclosed in double
         quotes (") in order to represent arguments containing spaces.
    
         The possible keywords and their meanings are as follows (note that key‐
         words are case-insensitive and arguments are case-sensitive):
    
         AcceptEnv
    	     Specifies what environment variables sent by the client will be
    	     copied into the session's environ(7).  See SendEnv in
    	     ssh_config(5) for how to configure the client.  Note that envi‐
    	     ronment passing is only supported for protocol 2, and that the
    	     TERM environment variable is always sent whenever the client
    	     requests a pseudo-terminal as it is required by the protocol.
    	     Variables are specified by name, which may contain the wildcard
    	     characters ‘*’ and ‘?’.  Multiple environment variables may be
    	     separated by whitespace or spread across multiple AcceptEnv
    	     directives.  Be warned that some environment variables could be
    	     used to bypass restricted user environments.  For this reason,
    	     care should be taken in the use of this directive.	 The default
    	     is not to accept any environment variables.
    
         AddressFamily
    	     Specifies which address family should be used by sshd(8).	Valid
    	     arguments are “any”, “inet” (use IPv4 only), or “inet6” (use IPv6
    	     only).  The default is “any”.
    
         AllowAgentForwarding
    	     Specifies whether ssh-agent(1) forwarding is permitted.  The
    	     default is “yes”.	Note that disabling agent forwarding does not
    	     improve security unless users are also denied shell access, as
    	     they can always install their own forwarders.
    
         AllowGroups
    	     This keyword can be followed by a list of group name patterns,
    	     separated by spaces.  If specified, login is allowed only for
    	     users whose primary group or supplementary group list matches one
    	     of the patterns.  Only group names are valid; a numerical group
    	     ID is not recognized.  By default, login is allowed for all
    	     groups.  The allow/deny directives are processed in the following
    	     order: DenyUsers, AllowUsers, DenyGroups, and finally
    	     AllowGroups.  All of the specified user and group tests must suc‐
    	     ceed, before user is allowed to log in.
    
    	     See PATTERNS in ssh_config(5) for more information on patterns.
    
         AllowTcpForwarding
    	     Specifies whether TCP forwarding is permitted.  The available
    	     options are “yes” or “all” to allow TCP forwarding, “no” to pre‐
    	     vent all TCP forwarding, “local” to allow local (from the per‐
    	     spective of ssh(1)) forwarding only or “remote” to allow remote
    	     forwarding only.  The default is “yes”.  Note that disabling TCP
    	     forwarding does not improve security unless users are also denied
    	     shell access, as they can always install their own forwarders.
    
         AllowUsers
    	     This keyword can be followed by a list of user name patterns,
    	     separated by spaces.  If specified, login is allowed only for
    	     user names that match one of the patterns.	 Only user names are
    	     valid; a numerical user ID is not recognized.  By default, login
    	     is allowed for all users.	If the pattern takes the form
    	     USER@HOST then USER and HOST are separately checked, restricting
    	     logins to particular users from particular hosts.	The allow/deny
    	     directives are processed in the following order: DenyUsers,
    	     AllowUsers, DenyGroups, and finally AllowGroups.  All of the
    	     specified user and group tests must succeed, before user is
    	     allowed to log in.
    
    	     See PATTERNS in ssh_config(5) for more information on patterns.
    
         AuthenticationMethods
    	     Specifies the authentication methods that must be successfully
    	     completed for a user to be granted access.	 This option must be
    	     followed by one or more comma-separated lists of authentication
    	     method names, or by the single string “any” to indicate the
    	     default behaviour of accepting any single authentication method.
    	     if the default is overridden, then successful authentication
    	     requires completion of every method in at least one of these
    	     lists.
    
    	     For example, an argument of “publickey,password
    	     publickey,keyboard-interactive” would require the user to com‐
    	     plete public key authentication, followed by either password or
    	     keyboard interactive authentication.  Only methods that are next
    	     in one or more lists are offered at each stage, so for this exam‐
    	     ple, it would not be possible to attempt password or keyboard-
    	     interactive authentication before public key.
    
    	     For keyboard interactive authentication it is also possible to
    	     restrict authentication to a specific device by appending a colon
    	     followed by the device identifier “bsdauth”, “pam”, or “skey”,
    	     depending on the server configuration.  For example,
    	     “keyboard-interactive:bsdauth” would restrict keyboard interac‐
    	     tive authentication to the “bsdauth” device.
    
    	     This option is only available for SSH protocol 2 and will yield a
    	     fatal error if enabled if protocol 1 is also enabled.  Note that
    	     each authentication method listed should also be explicitly
    	     enabled in the configuration.  The default “any” is not to
    	     require multiple authentication; successful completion of a sin‐
    	     gle authentication method is sufficient.
    
         AuthorizedKeysCommand
    	     Specifies a program to be used to look up the user's public keys.
    	     The program must be owned by root and not writable by group or
    	     others.  It will be invoked with a single argument of the user‐
    	     name being authenticated, and should produce on standard output
    	     zero or more lines of authorized_keys output (see AUTHORIZED_KEYS
    	     in sshd(8)).  If a key supplied by AuthorizedKeysCommand does not
    	     successfully authenticate and authorize the user then public key
    	     authentication continues using the usual AuthorizedKeysFile
    	     files.  By default, no AuthorizedKeysCommand is run.
    
         AuthorizedKeysCommandUser
    	     Specifies the user under whose account the AuthorizedKeysCommand
    	     is run.  It is recommended to use a dedicated user that has no
    	     other role on the host than running authorized keys commands.
    
         AuthorizedKeysFile
    	     Specifies the file that contains the public keys that can be used
    	     for user authentication.  The format is described in the AUTHO‐
    	     RIZED_KEYS FILE FORMAT section of sshd(8).	 AuthorizedKeysFile
    	     may contain tokens of the form %T which are substituted during
    	     connection setup.	The following tokens are defined: %% is
    	     replaced by a literal '%', %h is replaced by the home directory
    	     of the user being authenticated, and %u is replaced by the user‐
    	     name of that user.	 After expansion, AuthorizedKeysFile is taken
    	     to be an absolute path or one relative to the user's home direc‐
    	     tory.  Multiple files may be listed, separated by whitespace.
    	     The default is “.ssh/authorized_keys .ssh/authorized_keys2”.
    
         AuthorizedPrincipalsFile
    	     Specifies a file that lists principal names that are accepted for
    	     certificate authentication.  When using certificates signed by a
    	     key listed in TrustedUserCAKeys, this file lists names, one of
    	     which must appear in the certificate for it to be accepted for
    	     authentication.  Names are listed one per line preceded by key
    	     options (as described in AUTHORIZED_KEYS FILE FORMAT in sshd(8)).
    	     Empty lines and comments starting with ‘#’ are ignored.
    
    	     AuthorizedPrincipalsFile may contain tokens of the form %T which
    	     are substituted during connection setup.  The following tokens
    	     are defined: %% is replaced by a literal '%', %h is replaced by
    	     the home directory of the user being authenticated, and %u is
    	     replaced by the username of that user.  After expansion,
    	     AuthorizedPrincipalsFile is taken to be an absolute path or one
    	     relative to the user's home directory.
    
    	     The default is “none”, i.e. not to use a principals file – in
    	     this case, the username of the user must appear in a certifi‐
    	     cate's principals list for it to be accepted.  Note that
    	     AuthorizedPrincipalsFile is only used when authentication pro‐
    	     ceeds using a CA listed in TrustedUserCAKeys and is not consulted
    	     for certification authorities trusted via ~/.ssh/authorized_keys,
    	     though the principals= key option offers a similar facility (see
    	     sshd(8) for details).
    
         Banner  The contents of the specified file are sent to the remote user
    	     before authentication is allowed.	If the argument is “none” then
    	     no banner is displayed.  This option is only available for proto‐
    	     col version 2.  By default, no banner is displayed.
    
         ChallengeResponseAuthentication
    	     Specifies whether challenge-response authentication is allowed
    	     (e.g. via PAM or though authentication styles supported in
    	     login.conf(5)) The default is “yes”.
    
         ChrootDirectory
    	     Specifies the pathname of a directory to chroot(2) to after
    	     authentication.  All components of the pathname must be root-
    	     owned directories that are not writable by any other user or
    	     group.  After the chroot, sshd(8) changes the working directory
    	     to the user's home directory.
    
    	     The pathname may contain the following tokens that are expanded
    	     at runtime once the connecting user has been authenticated: %% is
    	     replaced by a literal '%', %h is replaced by the home directory
    	     of the user being authenticated, and %u is replaced by the user‐
    	     name of that user.
    
    	     The ChrootDirectory must contain the necessary files and directo‐
    	     ries to support the user's session.  For an interactive session
    	     this requires at least a shell, typically sh(1), and basic /dev
    	     nodes such as null(4), zero(4), stdin(4), stdout(4), stderr(4),
    	     arandom(4) and tty(4) devices.  For file transfer sessions using
    	     “sftp”, no additional configuration of the environment is neces‐
    	     sary if the in-process sftp server is used, though sessions which
    	     use logging do require /dev/log inside the chroot directory (see
    	     sftp-server(8) for details).
    
    	     The default is not to chroot(2).
    
         Ciphers
    	     Specifies the ciphers allowed for protocol version 2.  Multiple
    	     ciphers must be comma-separated.  The supported ciphers are:
    
    	     “3des-cbc”, “aes128-cbc”, “aes192-cbc”, “aes256-cbc”,
    	     “aes128-ctr”, “aes192-ctr”, “aes256-ctr”,
    	     “[email protected]”, “[email protected]”, “arcfour128”,
    	     “arcfour256”, “arcfour”, “blowfish-cbc”, “cast128-cbc”, and
    	     “[email protected]”.
    
    	     The default is:
    
    		aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
    		[email protected],[email protected],
    		[email protected],
    		aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
    		aes256-cbc,arcfour
    
    	     The list of available ciphers may also be obtained using the -Q
    	     option of ssh(1).
    
         ClientAliveCountMax
    	     Sets the number of client alive messages (see below) which may be
    	     sent without sshd(8) receiving any messages back from the client.
    	     If this threshold is reached while client alive messages are
    	     being sent, sshd will disconnect the client, terminating the ses‐
    	     sion.  It is important to note that the use of client alive mes‐
    	     sages is very different from TCPKeepAlive (below).	 The client
    	     alive messages are sent through the encrypted channel and there‐
    	     fore will not be spoofable.  The TCP keepalive option enabled by
    	     TCPKeepAlive is spoofable.	 The client alive mechanism is valu‐
    	     able when the client or server depend on knowing when a connec‐
    	     tion has become inactive.
    
    	     The default value is 3.  If ClientAliveInterval (see below) is
    	     set to 15, and ClientAliveCountMax is left at the default, unre‐
    	     sponsive SSH clients will be disconnected after approximately 45
    	     seconds.  This option applies to protocol version 2 only.
    
         ClientAliveInterval
    	     Sets a timeout interval in seconds after which if no data has
    	     been received from the client, sshd(8) will send a message
    	     through the encrypted channel to request a response from the
    	     client.  The default is 0, indicating that these messages will
    	     not be sent to the client.	 This option applies to protocol ver‐
    	     sion 2 only.
    
         Compression
    	     Specifies whether compression is allowed, or delayed until the
    	     user has authenticated successfully.  The argument must be “yes”,
    	     “delayed”, or “no”.  The default is “delayed”.
    
         DenyGroups
    	     This keyword can be followed by a list of group name patterns,
    	     separated by spaces.  Login is disallowed for users whose primary
    	     group or supplementary group list matches one of the patterns.
    	     Only group names are valid; a numerical group ID is not recog‐
    	     nized.  By default, login is allowed for all groups.  The
    	     allow/deny directives are processed in the following order:
    	     DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups.  All
    	     of the specified user and group tests must succeed, before user
    	     is allowed to log in.
    
    	     See PATTERNS in ssh_config(5) for more information on patterns.
    
         DenyUsers
    	     This keyword can be followed by a list of user name patterns,
    	     separated by spaces.  Login is disallowed for user names that
    	     match one of the patterns.	 Only user names are valid; a numeri‐
    	     cal user ID is not recognized.  By default, login is allowed for
    	     all users.	 If the pattern takes the form USER@HOST then USER and
    	     HOST are separately checked, restricting logins to particular
    	     users from particular hosts.  The allow/deny directives are pro‐
    	     cessed in the following order: DenyUsers, AllowUsers, DenyGroups,
    	     and finally AllowGroups.  All of the specified user and group
    	     tests must succeed, before user is allowed to log in.
    
    	     See PATTERNS in ssh_config(5) for more information on patterns.
    
         ExposeAuthenticationMethods
    	     When using SSH2, this option controls the exposure of the list of
    	     successful authentication methods to PAM during the authentica‐
    	     tion and to the shell environment via the SSH_USER_AUTH variable.
    	     See the description of this variable for more details.  Valid
    	     options are: “never” (Do not expose successful authentication
    	     methods), “pam-only” (Only expose them to PAM during authentica‐
    	     tion, not afterwards), “pam-and-env” (Expose them to PAM and keep
    	     them in the shell environment).  The default is “never”.
    
         ForceCommand
    	     Forces the execution of the command specified by ForceCommand,
    	     ignoring any command supplied by the client and ~/.ssh/rc if
    	     present.  The command is invoked by using the user's login shell
    	     with the -c option.  This applies to shell, command, or subsystem
    	     execution.	 It is most useful inside a Match block.  The command
    	     originally supplied by the client is available in the
    	     SSH_ORIGINAL_COMMAND environment variable.	 Specifying a command
    	     of “internal-sftp” will force the use of an in-process sftp
    	     server that requires no support files when used with
    	     ChrootDirectory.
    
         GatewayPorts
    	     Specifies whether remote hosts are allowed to connect to ports
    	     forwarded for the client.	By default, sshd(8) binds remote port
    	     forwardings to the loopback address.  This prevents other remote
    	     hosts from connecting to forwarded ports.	GatewayPorts can be
    	     used to specify that sshd should allow remote port forwardings to
    	     bind to non-loopback addresses, thus allowing other hosts to con‐
    	     nect.  The argument may be “no” to force remote port forwardings
    	     to be available to the local host only, “yes” to force remote
    	     port forwardings to bind to the wildcard address, or
    	     “clientspecified” to allow the client to select the address to
    	     which the forwarding is bound.  The default is “no”.
    
         GSSAPIAuthentication
    	     Specifies whether user authentication based on GSSAPI is allowed.
    	     The default is “no”.  Note that this option applies to protocol
    	     version 2 only.
    
         GSSAPIKeyExchange
    	     Specifies whether key exchange based on GSSAPI is allowed. GSSAPI
    	     key exchange doesn't rely on ssh keys to verify host identity.
    	     The default is “no”.  Note that this option applies to protocol
    	     version 2 only.
    
         GSSAPICleanupCredentials
    	     Specifies whether to automatically destroy the user's credentials
    	     cache on logout.  The default is “yes”.  Note that this option
    	     applies to protocol version 2 only.
    
         GSSAPIEnablek5users
    	     Specifies whether to look at .k5users file for GSSAPI authentica‐
    	     tion access control. Further details are described in ksu(1).
    	     The default is “no”.
    
         GSSAPIStrictAcceptorCheck
    	     Determines whether to be strict about the identity of the GSSAPI
    	     acceptor a client authenticates against. If “yes” then the client
    	     must authenticate against the host service on the current host‐
    	     name. If “no” then the client may authenticate against any ser‐
    	     vice key stored in the machine's default store. This facility is
    	     provided to assist with operation on multi homed machines.	 The
    	     default is “yes”.	Note that this option applies only to protocol
    	     version 2 GSSAPI connections, and setting it to “no” may only
    	     work with recent Kerberos GSSAPI libraries.
    
         GSSAPIStoreCredentialsOnRekey
    	     Controls whether the user's GSSAPI credentials should be updated
    	     following a successful connection rekeying. This option can be
    	     used to accepted renewed or updated credentials from a compatible
    	     client. The default is “no”.
    
         GSSAPIKexAlgorithms
    	     The list of key exchange algorithms that are accepted by GSSAPI
    	     key exchange. Possible values are
    
    		gss-gex-sha1-,
    		gss-group1-sha1-,
    		gss-group14-sha1-
    
    	     The default is
    	     “gss-gex-sha1-,gss-group1-sha1-,gss-group14-sha1-”.  This option
    	     only applies to protocol version 2 connections using GSSAPI.
    
         HostbasedAuthentication
    	     Specifies whether rhosts or /etc/hosts.equiv authentication
    	     together with successful public key client host authentication is
    	     allowed (host-based authentication).  This option is similar to
    	     RhostsRSAAuthentication and applies to protocol version 2 only.
    	     The default is “no”.
    
         HostbasedUsesNameFromPacketOnly
    	     Specifies whether or not the server will attempt to perform a
    	     reverse name lookup when matching the name in the ~/.shosts,
    	     ~/.rhosts, and /etc/hosts.equiv files during
    	     HostbasedAuthentication.  A setting of “yes” means that sshd(8)
    	     uses the name supplied by the client rather than attempting to
    	     resolve the name from the TCP connection itself.  The default is
    	     “no”.
    
         HostCertificate
    	     Specifies a file containing a public host certificate.  The cer‐
    	     tificate's public key must match a private host key already spec‐
    	     ified by HostKey.	The default behaviour of sshd(8) is not to
    	     load any certificates.
    
         HostKey
    	     Specifies a file containing a private host key used by SSH.  The
    	     default is /etc/ssh/ssh_host_key for protocol version 1, and
    	     /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_ecdsa_key,
    	     /etc/ssh/ssh_host_ed25519_key and /etc/ssh/ssh_host_rsa_key for
    	     protocol version 2.  Note that sshd(8) will refuse to use a file
    	     if it is group/world-accessible.  It is possible to have multiple
    	     host key files.  “rsa1” keys are used for version 1 and “dsa”,
    	     “ecdsa”, “ed25519” or “rsa” are used for version 2 of the SSH
    	     protocol.	It is also possible to specify public host key files
    	     instead.  In this case operations on the private key will be del‐
    	     egated to an ssh-agent(1).
    
         HostKeyAgent
    	     Identifies the UNIX-domain socket used to communicate with an
    	     agent that has access to the private host keys.  If
    	     “SSH_AUTH_SOCK” is specified, the location of the socket will be
    	     read from the SSH_AUTH_SOCK environment variable.
    
         IgnoreRhosts
    	     Specifies that .rhosts and .shosts files will not be used in
    	     RhostsRSAAuthentication or HostbasedAuthentication.
    
    	     /etc/hosts.equiv and /etc/ssh/shosts.equiv are still used.	 The
    	     default is “yes”.
    
         IgnoreUserKnownHosts
    	     Specifies whether sshd(8) should ignore the user's
    	     ~/.ssh/known_hosts during RhostsRSAAuthentication or
    	     HostbasedAuthentication.  The default is “no”.
    
         IPQoS   Specifies the IPv4 type-of-service or DSCP class for the connec‐
    	     tion.  Accepted values are “af11”, “af12”, “af13”, “af21”,
    	     “af22”, “af23”, “af31”, “af32”, “af33”, “af41”, “af42”, “af43”,
    	     “cs0”, “cs1”, “cs2”, “cs3”, “cs4”, “cs5”, “cs6”, “cs7”, “ef”,
    	     “lowdelay”, “throughput”, “reliability”, or a numeric value.
    	     This option may take one or two arguments, separated by white‐
    	     space.  If one argument is specified, it is used as the packet
    	     class unconditionally.  If two values are specified, the first is
    	     automatically selected for interactive sessions and the second
    	     for non-interactive sessions.  The default is “lowdelay” for
    	     interactive sessions and “throughput” for non-interactive ses‐
    	     sions.
    
         KbdInteractiveAuthentication
    	     Specifies whether to allow keyboard-interactive authentication.
    	     The argument to this keyword must be “yes” or “no”.  The default
    	     is to use whatever value ChallengeResponseAuthentication is set
    	     to (by default “yes”).
    
         KerberosAuthentication
    	     Specifies whether the password provided by the user for
    	     PasswordAuthentication will be validated through the Kerberos
    	     KDC.  To use this option, the server needs a Kerberos servtab
    	     which allows the verification of the KDC's identity.  The default
    	     is “no”.
    
         KerberosGetAFSToken
    	     If AFS is active and the user has a Kerberos 5 TGT, attempt to
    	     acquire an AFS token before accessing the user's home directory.
    	     The default is “no”.
    
         KerberosOrLocalPasswd
    	     If password authentication through Kerberos fails then the pass‐
    	     word will be validated via any additional local mechanism such as
    	     /etc/passwd.  The default is “yes”.
    
         KerberosTicketCleanup
    	     Specifies whether to automatically destroy the user's ticket
    	     cache file on logout.  The default is “yes”.
    
         KerberosUseKuserok
    	     Specifies whether to look at .k5login file for user's aliases.
    	     The default is “yes”.
    
         KexAlgorithms
    	     Specifies the available KEX (Key Exchange) algorithms.  Multiple
    	     algorithms must be comma-separated.  The default is
    
    		   [email protected],
    		   ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
    		   diffie-hellman-group-exchange-sha256,
    		   diffie-hellman-group-exchange-sha1,
    		   diffie-hellman-group14-sha1,
    		   diffie-hellman-group1-sha1
    
         KeyRegenerationInterval
    	     In protocol version 1, the ephemeral server key is automatically
    	     regenerated after this many seconds (if it has been used).	 The
    	     purpose of regeneration is to prevent decrypting captured ses‐
    	     sions by later breaking into the machine and stealing the keys.
    	     The key is never stored anywhere.	If the value is 0, the key is
    	     never regenerated.	 The default is 3600 (seconds).
    
         ListenAddress
    	     Specifies the local addresses sshd(8) should listen on.  The fol‐
    	     lowing forms may be used:
    
    		   ListenAddress host|IPv4_addr|IPv6_addr
    		   ListenAddress host|IPv4_addr:port
    		   ListenAddress [host|IPv6_addr]:port
    
    	     If port is not specified, sshd will listen on the address and all
    	     prior Port options specified.  The default is to listen on all
    	     local addresses.  Multiple ListenAddress options are permitted.
    	     Additionally, any Port options must precede this option for non-
    	     port qualified addresses.
    
         LoginGraceTime
    	     The server disconnects after this time if the user has not suc‐
    	     cessfully logged in.  If the value is 0, there is no time limit.
    	     The default is 120 seconds.
    
         LogLevel
    	     Gives the verbosity level that is used when logging messages from
    	     sshd(8).  The possible values are: QUIET, FATAL, ERROR, INFO,
    	     VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.  The default is INFO.
    	     DEBUG and DEBUG1 are equivalent.  DEBUG2 and DEBUG3 each specify
    	     higher levels of debugging output.	 Logging with a DEBUG level
    	     violates the privacy of users and is not recommended.
    
         MACs    Specifies the available MAC (message authentication code) algo‐
    	     rithms.  The MAC algorithm is used in protocol version 2 for data
    	     integrity protection.  Multiple algorithms must be comma-sepa‐
    	     rated.  The algorithms that contain “-etm” calculate the MAC
    	     after encryption (encrypt-then-mac).  These are considered safer
    	     and their use recommended.	 The default is:
    
    		   [email protected],[email protected],
    		   [email protected],[email protected],
    		   [email protected],[email protected],
    		   [email protected],[email protected],
    		   [email protected],
    		   hmac-md5,hmac-sha1,[email protected],[email protected],
    		   hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,
    		   hmac-sha1-96,hmac-md5-96
    
         Match   Introduces a conditional block.  If all of the criteria on the
    	     Match line are satisfied, the keywords on the following lines
    	     override those set in the global section of the config file,
    	     until either another Match line or the end of the file.  If a
    	     keyword appears in multiple Match blocks that are satisified,
    	     only the first instance of the keyword is applied.
    
    	     The arguments to Match are one or more criteria-pattern pairs or
    	     the single token All which matches all criteria.  The available
    	     criteria are User, Group, Host, LocalAddress, LocalPort, and
    	     Address.  The match patterns may consist of single entries or
    	     comma-separated lists and may use the wildcard and negation oper‐
    	     ators described in the PATTERNS section of ssh_config(5).
    
    	     The patterns in an Address criteria may additionally contain
    	     addresses to match in CIDR address/masklen format, e.g.
    	     “192.0.2.0/24” or “3ffe:ffff::/32”.  Note that the mask length
    	     provided must be consistent with the address - it is an error to
    	     specify a mask length that is too long for the address or one
    	     with bits set in this host portion of the address.	 For example,
    	     “192.0.2.0/33” and “192.0.2.0/8” respectively.
    
    	     Only a subset of keywords may be used on the lines following a
    	     Match keyword.  Available keywords are AcceptEnv,
    	     AllowAgentForwarding, AllowGroups, AllowTcpForwarding,
    	     AllowUsers, AuthenticationMethods, AuthorizedKeysCommand,
    	     AuthorizedKeysCommandUser, AuthorizedKeysFile,
    	     AuthorizedPrincipalsFile, Banner, ChrootDirectory, DenyGroups,
    	     DenyUsers, ForceCommand, GatewayPorts, GSSAPIAuthentication,
    	     HostbasedAuthentication, HostbasedUsesNameFromPacketOnly,
    	     KbdInteractiveAuthentication, KerberosAuthentication,
    	     KerberosUseKuserok, MaxAuthTries, MaxSessions,
    	     PasswordAuthentication, PermitEmptyPasswords, PermitOpen,
    	     PermitRootLogin, PermitTTY, PermitTunnel, PubkeyAuthentication,
    	     RekeyLimit, RhostsRSAAuthentication, RSAAuthentication,
    	     X11DisplayOffset, X11MaxDisplays, X11Forwarding and
    	     X11UseLocalHost.
    
         MaxAuthTries
    	     Specifies the maximum number of authentication attempts permitted
    	     per connection.  Once the number of failures reaches half this
    	     value, additional failures are logged.  The default is 6.
    
         MaxSessions
    	     Specifies the maximum number of open sessions permitted per net‐
    	     work connection.  The default is 10.
    
         MaxStartups
    	     Specifies the maximum number of concurrent unauthenticated con‐
    	     nections to the SSH daemon.  Additional connections will be
    	     dropped until authentication succeeds or the LoginGraceTime
    	     expires for a connection.	The default is 10:30:100.
    
    	     Alternatively, random early drop can be enabled by specifying the
    	     three colon separated values “start:rate:full” (e.g. "10:30:60").
    	     sshd(8) will refuse connection attempts with a probability of
    	     “rate/100” (30%) if there are currently “start” (10) unauthenti‐
    	     cated connections.	 The probability increases linearly and all
    	     connection attempts are refused if the number of unauthenticated
    	     connections reaches “full” (60).
    
         PasswordAuthentication
    	     Specifies whether password authentication is allowed.  The
    	     default is “yes”.
    
         PermitEmptyPasswords
    	     When password authentication is allowed, it specifies whether the
    	     server allows login to accounts with empty password strings.  The
    	     default is “no”.
    
         PermitOpen
    	     Specifies the destinations to which TCP port forwarding is per‐
    	     mitted.  The forwarding specification must be one of the follow‐
    	     ing forms:
    
    		   PermitOpen host:port
    		   PermitOpen IPv4_addr:port
    		   PermitOpen [IPv6_addr]:port
    
    	     Multiple forwards may be specified by separating them with white‐
    	     space.  An argument of “any” can be used to remove all restric‐
    	     tions and permit any forwarding requests.	An argument of “none”
    	     can be used to prohibit all forwarding requests.  Wildcard “*”
    	     can be used for host or port to allow all hosts or all ports
    	     respectively.  By default all port forwarding requests are per‐
    	     mitted.
    
         PermitRootLogin
    	     Specifies whether root can log in using ssh(1).  The argument
    	     must be “yes”, “without-password”, “forced-commands-only”, or
    	     “no”.  The default is “yes”.
    
    	     If this option is set to “without-password”, password authentica‐
    	     tion is disabled for root.
    
    	     If this option is set to “forced-commands-only”, root login with
    	     public key authentication will be allowed, but only if the
    	     command option has been specified (which may be useful for taking
    	     remote backups even if root login is normally not allowed).  All
    	     other authentication methods are disabled for root.
    
    	     If this option is set to “no”, root is not allowed to log in.
    
         PermitTunnel
    	     Specifies whether tun(4) device forwarding is allowed.  The argu‐
    	     ment must be “yes”, “point-to-point” (layer 3), “ethernet” (layer
    	     2), or “no”.  Specifying “yes” permits both “point-to-point” and
    	     “ethernet”.  The default is “no”.
    
         PermitTTY
    	     Specifies whether pty(4) allocation is permitted.	The default is
    	     “yes”.
    
         PermitUserEnvironment
    	     Specifies whether ~/.ssh/environment and environment= options in
    	     ~/.ssh/authorized_keys are processed by sshd(8).  The default is
    	     “no”.  Enabling environment processing may enable users to bypass
    	     access restrictions in some configurations using mechanisms such
    	     as LD_PRELOAD.
    
         PidFile
    	     Specifies the file that contains the process ID of the SSH dae‐
    	     mon.  The default is /var/run/sshd.pid.
    
         Port    Specifies the port number that sshd(8) listens on.	 The default
    	     is 22.  Multiple options of this type are permitted.  See also
    	     ListenAddress.
    
         PrintLastLog
    	     Specifies whether sshd(8) should print the date and time of the
    	     last user login when a user logs in interactively.	 The default
    	     is “yes”.
    
         PrintMotd
    	     Specifies whether sshd(8) should print /etc/motd when a user logs
    	     in interactively.	(On some systems it is also printed by the
    	     shell, /etc/profile, or equivalent.)  The default is “yes”.
    
         Protocol
    	     Specifies the protocol versions sshd(8) supports.	The possible
    	     values are ‘1’ and ‘2’.  Multiple versions must be comma-sepa‐
    	     rated.  The default is ‘2’.  Note that the order of the protocol
    	     list does not indicate preference, because the client selects
    	     among multiple protocol versions offered by the server.  Specify‐
    	     ing “2,1” is identical to “1,2”.
    
         PubkeyAuthentication
    	     Specifies whether public key authentication is allowed.  The
    	     default is “yes”.	Note that this option applies to protocol ver‐
    	     sion 2 only.
    
         RekeyLimit
    	     Specifies the maximum amount of data that may be transmitted
    	     before the session key is renegotiated, optionally followed a
    	     maximum amount of time that may pass before the session key is
    	     renegotiated.  The first argument is specified in bytes and may
    	     have a suffix of ‘K’, ‘M’, or ‘G’ to indicate Kilobytes,
    	     Megabytes, or Gigabytes, respectively.  The default is between
    	     ‘1G’ and ‘4G’, depending on the cipher.  The optional second
    	     value is specified in seconds and may use any of the units docu‐
    	     mented in the TIME FORMATS section.  The default value for
    	     RekeyLimit is “default none”, which means that rekeying is per‐
    	     formed after the cipher's default amount of data has been sent or
    	     received and no time based rekeying is done.  This option applies
    	     to protocol version 2 only.
    
         RevokedKeys
    	     Specifies revoked public keys.  Keys listed in this file will be
    	     refused for public key authentication.  Note that if this file is
    	     not readable, then public key authentication will be refused for
    	     all users.	 Keys may be specified as a text file, listing one
    	     public key per line, or as an OpenSSH Key Revocation List (KRL)
    	     as generated by ssh-keygen(1).  For more information on KRLs, see
    	     the KEY REVOCATION LISTS section in ssh-keygen(1).
    
         RhostsRSAAuthentication
    	     Specifies whether rhosts or /etc/hosts.equiv authentication
    	     together with successful RSA host authentication is allowed.  The
    	     default is “no”.  This option applies to protocol version 1 only.
    
         RSAAuthentication
    	     Specifies whether pure RSA authentication is allowed.  The
    	     default is “yes”.	This option applies to protocol version 1
    	     only.
    
         ServerKeyBits
    	     Defines the number of bits in the ephemeral protocol version 1
    	     server key.  The minimum value is 512, and the default is 1024.
    
         ShowPatchLevel
    	     Specifies whether sshd will display the patch level of the binary
    	     in the identification string.  The patch level is set at compile-
    	     time.  The default is “no”.  This option applies to protocol ver‐
    	     sion 1 only.
    
         StrictModes
    	     Specifies whether sshd(8) should check file modes and ownership
    	     of the user's files and home directory before accepting login.
    	     This is normally desirable because novices sometimes accidentally
    	     leave their directory or files world-writable.  The default is
    	     “yes”.  Note that this does not apply to ChrootDirectory, whose
    	     permissions and ownership are checked unconditionally.
    
         Subsystem
    	     Configures an external subsystem (e.g. file transfer daemon).
    	     Arguments should be a subsystem name and a command (with optional
    	     arguments) to execute upon subsystem request.
    
    	     The command sftp-server(8) implements the “sftp” file transfer
    	     subsystem.
    
    	     Alternately the name “internal-sftp” implements an in-process
    	     “sftp” server.  This may simplify configurations using
    	     ChrootDirectory to force a different filesystem root on clients.
    
    	     By default no subsystems are defined.  Note that this option
    	     applies to protocol version 2 only.
    
         SyslogFacility
    	     Gives the facility code that is used when logging messages from
    	     sshd(8).  The possible values are: DAEMON, USER, AUTH, AUTHPRIV,
    	     LOCAL0, LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
    	     The default is AUTH.
    
         TCPKeepAlive
    	     Specifies whether the system should send TCP keepalive messages
    	     to the other side.	 If they are sent, death of the connection or
    	     crash of one of the machines will be properly noticed.  However,
    	     this means that connections will die if the route is down tempo‐
    	     rarily, and some people find it annoying.	On the other hand, if
    	     TCP keepalives are not sent, sessions may hang indefinitely on
    	     the server, leaving “ghost” users and consuming server resources.
    
    	     The default is “yes” (to send TCP keepalive messages), and the
    	     server will notice if the network goes down or the client host
    	     crashes.  This avoids infinitely hanging sessions.
    
    	     To disable TCP keepalive messages, the value should be set to
    	     “no”.
    
         TrustedUserCAKeys
    	     Specifies a file containing public keys of certificate authori‐
    	     ties that are trusted to sign user certificates for authentica‐
    	     tion.  Keys are listed one per line; empty lines and comments
    	     starting with ‘#’ are allowed.  If a certificate is presented for
    	     authentication and has its signing CA key listed in this file,
    	     then it may be used for authentication for any user listed in the
    	     certificate's principals list.  Note that certificates that lack
    	     a list of principals will not be permitted for authentication
    	     using TrustedUserCAKeys.  For more details on certificates, see
    	     the CERTIFICATES section in ssh-keygen(1).
    
         UseDNS  Specifies whether sshd(8) should look up the remote host name and
    	     check that the resolved host name for the remote IP address maps
    	     back to the very same IP address.	The default is “yes”.
    
         UseLogin
    	     Specifies whether login(1) is used for interactive login ses‐
    	     sions.  The default is “no”.  Note that login(1) is never used
    	     for remote command execution.  Note also, that if this is
    	     enabled, X11Forwarding will be disabled because login(1) does not
    	     know how to handle xauth(1) cookies.  If UsePrivilegeSeparation
    	     is specified, it will be disabled after authentication.
    
         UsePAM  Enables the Pluggable Authentication Module interface.  If set to
    	     “yes” this will enable PAM authentication using
    	     ChallengeResponseAuthentication and PasswordAuthentication in
    	     addition to PAM account and session module processing for all
    	     authentication types.
    
    	     Because PAM challenge-response authentication usually serves an
    	     equivalent role to password authentication, you should disable
    	     either PasswordAuthentication or ChallengeResponseAuthentication.
    
    	     If UsePAM is enabled, you will not be able to run sshd(8) as a
    	     non-root user.  The default is “no”.
    
         UsePrivilegeSeparation
    	     Specifies whether sshd(8) separates privileges by creating an
    	     unprivileged child process to deal with incoming network traffic.
    	     After successful authentication, another process will be created
    	     that has the privilege of the authenticated user.	The goal of
    	     privilege separation is to prevent privilege escalation by con‐
    	     taining any corruption within the unprivileged processes.	The
    	     default is “yes”.	If UsePrivilegeSeparation is set to “sandbox”
    	     then the pre-authentication unprivileged process is subject to
    	     additional restrictions.
    
         VersionAddendum
    	     Optionally specifies additional text to append to the SSH proto‐
    	     col banner sent by the server upon connection.  The default is
    	     “none”.
    
         X11DisplayOffset
    	     Specifies the first display number available for sshd(8)'s X11
    	     forwarding.  This prevents sshd from interfering with real X11
    	     servers.  The default is 10.
    
         X11MaxDisplays
    	     Specifies the maximum number of displays available for sshd(8)'s
    	     X11 forwarding.  This prevents sshd from exhausting local ports.
    	     The default is 1000.
    
         X11Forwarding
    	     Specifies whether X11 forwarding is permitted.  The argument must
    	     be “yes” or “no”.	The default is “no”.
    
    	     When X11 forwarding is enabled, there may be additional exposure
    	     to the server and to client displays if the sshd(8) proxy display
    	     is configured to listen on the wildcard address (see
    	     X11UseLocalhost below), though this is not the default.  Addi‐
    	     tionally, the authentication spoofing and authentication data
    	     verification and substitution occur on the client side.  The
    	     security risk of using X11 forwarding is that the client's X11
    	     display server may be exposed to attack when the SSH client
    	     requests forwarding (see the warnings for ForwardX11 in
    	     ssh_config(5)).  A system administrator may have a stance in
    	     which they want to protect clients that may expose themselves to
    	     attack by unwittingly requesting X11 forwarding, which can war‐
    	     rant a “no” setting.
    
    	     Note that disabling X11 forwarding does not prevent users from
    	     forwarding X11 traffic, as users can always install their own
    	     forwarders.  X11 forwarding is automatically disabled if UseLogin
    	     is enabled.
    
         X11UseLocalhost
    	     Specifies whether sshd(8) should bind the X11 forwarding server
    	     to the loopback address or to the wildcard address.  By default,
    	     sshd binds the forwarding server to the loopback address and sets
    	     the hostname part of the DISPLAY environment variable to
    	     “localhost”.  This prevents remote hosts from connecting to the
    	     proxy display.  However, some older X11 clients may not function
    	     with this configuration.  X11UseLocalhost may be set to “no” to
    	     specify that the forwarding server should be bound to the wild‐
    	     card address.  The argument must be “yes” or “no”.	 The default
    	     is “yes”.
    
         XAuthLocation
    	     Specifies the full pathname of the xauth(1) program.  The default
    	     is /usr/bin/xauth.
    
    TIME FORMATS
         sshd(8) command-line arguments and configuration file options that spec‐
         ify time may be expressed using a sequence of the form: time[qualifier],
         where time is a positive integer value and qualifier is one of the fol‐
         lowing:
    
    	   ⟨none⟩  seconds
    	   s | S   seconds
    	   m | M   minutes
    	   h | H   hours
    	   d | D   days
    	   w | W   weeks
    
         Each member of the sequence is added together to calculate the total time
         value.
    
         Time format examples:
    
    	   600	   600 seconds (10 minutes)
    	   10m	   10 minutes
    	   1h30m   1 hour 30 minutes (90 minutes)
    
    FILES
         /etc/ssh/sshd_config
    	     Contains configuration data for sshd(8).  This file should be
    	     writable by root only, but it is recommended (though not neces‐
    	     sary) that it be world-readable.
    
    SEE ALSO
         sshd(8)
    
    AUTHORS
         OpenSSH is a derivative of the original and free ssh 1.2.12 release by
         Tatu Ylonen.  Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
         de Raadt and Dug Song removed many bugs, re-added newer features and cre‐
         ated OpenSSH.  Markus Friedl contributed the support for SSH protocol
         versions 1.5 and 2.0.  Niels Provos and Markus Friedl contributed support
         for privilege separation.
    
    BSD			       January 27, 2018				   BSD
    

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